This invention relates to ink jet printing. More specifically, it relates to printing using multiple nozzle, continuous drop printers in which a plurality of ink jets are provided in an array. In order to operate the nozzles, it is necessary to correctly phase the jet streams before printing. This is accomplished in a setup or phasing mode of operation prior to the initiation of printing operation. Phasing is done by time shifting the charge pulse supplied to the charging electrode associated with each jet stream and measuring the charge induced on the ink drops. This data is used by the micro-processor based controller to optimize drop charging.
Due to changes in operating temperature, ink viscosity, pressure, as well as other environmental factors, it has been necessary not only to phase the printing array before printing begins, but to periodically stop printing to rephase the array. Because such printers are typically used in high output environments, where it is necessary to maximize printing, this interruption for phasing purposes is undesirable. More specifically, such printers are often used to mark products on assembly lines and the like with date codes, product codes and similar information. The use of these printers on high speed assembly lines can be tolerated only if the printers are both reliable and capable of providing high quality marking over extended periods of operation under changing environmental conditions.
It is accordingly an object of the present invention to provide an improved method and apparatus which can avoid the need for periodic interruption of the printing process to rephasing the jets.
It is another object of the invention to provide a printing device which continuously monitors phasing during the printing operation and which is capable of adjusting phase to maintain optimal printing without interrupting printing.
A further object of the invention is to provide a separate pilot nozzle for a printer of the type described which continuously monitors changes in phase due to environmental factors, thereby to permit automatic correction as changes in phase occur.
These and other objects of the invention will be apparent from the remaining portion of the specification.